I try not to think about it. My list of regrets is relatively short for a collecting career that is now standing at 35 years, but here goes:
*Forget how I acquired it, but sold a '52 Topps #312 Jackie, probably G-VG (perfect, and I mean perfectly centered...) but with a light vertical crease down the middle - almost the length of the card - did not break the surface - one of those "twist in the light to see" issues - for $500 or so probably along about 2002.
*E92 Dockman Cy Young. I bought at a show also in the early 2000's for maybe $500 raw. I did not know much about E cards at the time, and this proved to be my downfall. I somehow convinced myself the card was trimmed, and sold it on eBay. I made my money back or took a small loss. About 6 months later I saw the same card, and I know it was mine by identifying marks - in a PSA 5 slab where it then sold for about 4x what I had in it at the time. Who knows what that would go for today...
*A nice and centered PSA 5 '53 Topps Satchel Paige that I paid $420 I think for in 2017. I sold it about a year and a half later for a modest profit. Poof...well, that's worth a ton more now.
Anymore I keep coming back to the same lesson learned: Just don't sell cards. Do you really have to? Be bored for awhile. It's sometimes better than selling something to finance whatever it is you have to have in the moment...only to to realize you F'd up later.
